The easy answer that most people might toss out there is that every group is different, but that’s hardly useful. The truth is, there are some good rules to follow that make most games fun.
1) First is the story arc. Even the most hack-n’-slash groups need a reason to delve into that deep, dank dungeon, and the order of monsters encountered in there will make the game either fun or boring. You need the smaller monsters first, then the big baddie at the end. It has to build. This rule is unbendable 99% of the time. The same goes for a game story. Start out innocuous, then build tension and conflict until the big finish and reward at the end.
2) To make it fun, you need a sprinkling of silliness. Sometimes people look up and say, “hey, I’m playing a game, and it’s funny!” Allow this to happen. However, don’t let an entire session or game story devolve into jokes and goblins in go-karts (TM). Let the story take a silly turn for an interlude, and give everyone’s brain a break from trying to figure things out and dungeon delving. Then, when the laughter dies down, begin building a dramatic arc once more.
3) Don’t allow game imbalances to derail your game. If you notice a player outperforming the others, you don’t have to mention it. Let the game roll on until a voice is raised against the injustice. You will exhaust yourself trying to make something perfect that is inherently interesting in its flawed state, which is the game design itself.
4) Nevertheless, be proactive. Put yourself forward to fix a problem before it happens, even if you don’t mention it. Throw yourself into a story, and inhabit a role as fully as you can, creating quirks of imagination as they come to you, and fitting them into the coherent world.
5) Congratulate others on creativity well done. Nothing makes a game good so much as communication of appreciation.
Good luck, and feel free to add commandments in the comments. 🙂